Line a cookie sheet with a silicone mat or parchment paper with 1 ½ inch circles drawn onto the paper.
Prepare the macarons. Sift the powdered sugar, almond flour, and instant espresso powder two times through a fine sieve.
Place the egg whites in the bowl of a stand mixer and whip with the whisk attachment until soft peaks form. Gradually add the sugar while the mixer is running and continue to whip until stiff peaks form. Add a third of the sifted dry ingredients to the egg whites, folding them into the egg whites with a spatula. Add the remaining dry ingredients in two more batches. Continue to gently fold the batter until the ingredients are fully combined. The batter is ready when you can hold the spatula over the bowl and draw a figure 8 smoothly without the batter breaking off (about 50-60 folds).
Transfer the batter to a piping bag and pipe 1 ½ inch discs onto the prepared cookie sheet. Firmly tap the cookie sheet onto the counter 3 or 4 times to bring any air bubbles to the surface. If there are any remaining bubbles, pop them with a toothpick.
Let the macarons sit out at room temperature until a skin forms on the top, about 1 hour.
Preheat the oven to 280 degrees F.
Bake the macarons in the center rack of the oven for 15 minutes, rotating the pan halfway through the baking time. Remove from the oven and cool completely on the baking sheet.
While the macarons are cooling, make the caramel buttercream. Beat the butter in the bowl of a stand mixer until softened, about 1 minute. Add the caramel and vanilla and beat for another minute or until well combined. Add the powdered sugar and the pinch of salt and continue to beat until smooth, about 2 minutes. Transfer the buttercream to a piping bag.
Peel the macarons off the baking sheet. Flip off half of the macarons and pipe the buttercream onto the macaron. Sandwich with another cookie and serve.